Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Fri Sep 25 2020

Valid 12Z Mon Sep 28 2020 - 12Z Fri Oct 02 2020

...Guidance/Predictability Assessment...

There is an increasing guidance signal supporting development of a
highly amplified pattern across the CONUS next week, with ridging
across the West and a deep upper trough/low over the central and
eastern U.S. This will lead to warm to hot temperatures once again
in the West, and cool temperatures more reminiscent of mid- to
late fall farther east. Rainfall should focus from the Great
Lakes/Mid-South toward the Appalachians and East Coast next week.
However, there remains quite a bit of forecast spread with respect
to the degree of flow separation within the main upper trough. The
12/18 UTC GFS and 12 UTC Canadian showed minimal flow separation
compared to the 12 UTC ECMWF/UKMET/NAVGEM that even show a closed
low development threat. I placed emphasis on the 12 UTC ECMWF
ensemble mean for the WPC medium range product suite that leaned
2/3 the way toward the more amplified solution composite given
upstream guidance amplification trends. Newer 00 UTC guidance has
trended this way as well, but still leaves the door open for
closed low development.

...Weather/Hazard Highlights...

Ample energy seems set to dig into the potent upper trough/low
position will push cold fronts across the central and eastern U.S.
through the period and provide focus for rain. Amplification
trends have slowed the eastward translation of a main lead front
and reinforcement as contingent on uncertain stream separation. A
significant WPC frontal continuity adjustment was applied to
address timing. Modest to locally heavy rainfall totals may focus
from the Appalachians eastward for the first part of the workweek,
albeit with trough axis differences affecting wave development and
moisture inflow potential. Florida will also see some rain along
the fronts as they try to push across the state and bring drier
air. Unsettled conditions are expected to linger across the
Midwest/Mid-South to Great Lakes region under the upper trough.

Given the upper-level pattern and the cold fronts, below normal
temperatures are forecast to push across the Plains toward the
Eastern Seaboard, especially in terms of daytime highs, which
should be 5-15 degrees F below normal. But by Thu/Fri, below
average lows are also forecast for the Central/Northern Plains
toward the Midwest, and there is a chance for a first frost or
freeze of the season in portions of those areas.

Upper ridge re-emergence will return hot, dry conditions to the
West, so fire danger is a concern. Much above normal high
temperatures are forecast to build from the Desert Southwest and
California/Nevada to the Pacific Northwest. Some record-breaking
highs/warm lows look to be approached/exceeded especially over
parts of California.

Schichtel

Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml